linux-kernel - Re: [PATCH 3/3] sched, fair: Stop searching for tasks in newidle balance if there are runnable tasks

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Message-ID: <1398404806.7893.18.camel@marge.simpson.net>
Date:	Fri, 25 Apr 2014 07:46:46 +0200
From:	Mike Galbraith <umgwanakikbuti@...il.com>
To:	Jason Low <jason.low2@...com>
Cc:	Peter Zijlstra <peterz@...radead.org>, mingo@...nel.org,
	linux-kernel@...r.kernel.org, daniel.lezcano@...aro.org,
	alex.shi@...aro.org, preeti@...ux.vnet.ibm.com,
	vincent.guittot@...aro.org, morten.rasmussen@....com, aswin@...com,
	chegu_vinod@...com
Subject: Re: [PATCH 3/3] sched, fair: Stop searching for tasks in newidle
 balance if there are runnable tasks

On Thu, 2014-04-24 at 20:33 -0700, Jason Low wrote: 
> On Fri, 2014-04-25 at 04:45 +0200, Mike Galbraith wrote:
> > On Thu, 2014-04-24 at 18:52 +0200, Peter Zijlstra wrote: 
> > > On Thu, Apr 24, 2014 at 09:43:09AM -0700, Jason Low wrote:
> > > > If the below patch is what you were referring to, I believe this
> > > > can help too. This was also something that I was testing out before
> > > > we went with those patches which compares avg_idle with idle balance
> > > > cost. I recall seeing somewhere around a +7% performance improvement
> > > > in at least least 1 of the AIM7 workloads. I can do some more testing
> > > > with this.
> > > 
> > > Yes, exactly that.
> > > 
> > > I can't remember the details, but I suspect we feared the less agressive
> > > idle balance due to that patch (it will only pull a single task, instead
> > > of multiple) would cause more idle_balance invocations and thereby
> > > decrease throughput.
> > > 
> > > So I suppose something with _many_ bursty threads which leads to severe
> > > inequalities would be the workload to trigger that.
> > > 
> > > Not sure we've ever seen that.. maybe Mike remembers, he seems to have a
> > > head for such details.
> > 
> > I don't recall ever seeing such.
> 
> Hmmm, could commit: 1b9508f6831e (sched: Rate-limit newidle) be related
> to what Peter's referring to? The patch mentioned that the rate-limit
> benefited "sysbench oltp".

That's the opposite, was to prevent way too much idle balancing, which
was demolishing fast mover loads.

-Mike

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